1. Field of the Invention
This invention relates to a parallel speed regulator system for controlling the field excitation of a motor operated from a common bus electrical supply.
2. Description of the Prior Art
In bar, rod, and stretch mills, the mill motors are frequently operated on a common bus system, with many drive motors arranged on a single armature supply. In most prior art systems, speed regulation is accomplished by controlling motor field excitation by means of a multiloop arrangement comprising an inner field current loop, an armature current loop, and an outside speed loop. The principal reason for utilizing the multiloop system is to enable the inner feedback loops to linearize the regulating system. When a regulating system controls motor field excitation the results are inherently non-linear because the gain coefficients and time constants of the motor and load vary with the operating point of the motor. The non-linearities are the result of controlling field flux where motor torque is a function of field flux and armature current, and speed is a function of back emf divided by field flux. With the advent of static multipliers and dividers, it is now possible to provide both gain and time constant compensation which vary with the operating point of the regulator controllers. By using this technique it is possible to linearize the regulator feedback loop so that the system has the same dynamic response at all operating points.
As of this date there does not appear to be any successful parallel speed regulator operating on a common bus speed regulated drive system. The parallel speed regulator has several advantages over the prior art multiloop speed regulators. In the first place, the parallel system is faster, has less speed droop, and a smaller percentage speed error times seconds for a step impact load condition. This parallel system can be less sensitive to noise from tachometers and the like. Inherently there is no possibility of controller windup.
The present invention provides successful parallel speed regulator with all the inherent advantages vis-a-vis the multiloop control, together with compensation which varies as a function of the instantaneous operating point of the motor, so that the dynamics of the speed regulator remain the same regardless of shifts in operating point.